Can anyone tell me what the typical accuracy of a decent pair of vernier
calipers is (when used carefully and correctly)? I just bought a dial
bore gauge and am wondering if I can reasonably set the zero point with
vernier calipers instead of having to buy a 3" to 4" micrometer. The
calipers are Etalon brand, heavy and made in Switzerland. The bore gauge
dial is graduated down to 0.0005". I'm not reboring an engine, I just
want an idea of the wear, so I don't need the highest accuracy. From
using these calipers, my own guess is that they're probably good to
0.002", but some of that might be my errors in reading them. I'd be
interested to hear what other people think.

Keep them within a few degrees of some standard (68 deg. F being the most
common one); keep your hands off of them and wear gloves; develop a good
feel, and you should be able to measure to +/- 0.001" with them. Your Etalon
probably is like my TESA Swiss vernier caliper, which is my best one.

You can easily check it with a few gage blocks. If you're going to do
precision work, you need at least a few of them. A set is great but for
checking mikes, calipers, height gages and so on, you only need a few in
different sizes. Then you won't be chasing your tail. You'll know with good
assurance how accurate your gages are. And they'll help you develop a feel
for how much force to use when you're measuring. A couple of gage pins, like
1/4" and 5/8", or something like that, are also useful for that purpose. I
have some precision toolmaker's buttons that serve that purpose.

This is the kind I have, except mine are 15" calipers and they don't
carry the P. Roch name:
http://cgi.ebay.com/ETALON-P-ROCH-ROLLE-SUISSE-12-30-mm-VERNIER-CALIPER_W0QQitemZ270531036164QQcmdZViewItemQQptZLH_DefaultDomain_0?hash=item3efce80404

Why do you suggest wearing gloves? To keep the calipers cool? Right now
the workshop is more like 40 deg. F.

I don't have any gauge blocks. Do you think they'd be a better
investment than a 3" to 4" micrometer?

The import 1-2-3 blocks are $10 at Enco. Then you have standards for whole
inches 1 through 6 to better than 0.001". Add a granite slab and you have
the beginnings of some serious metrology for all of $35. Just buy $15 more
worth of anything and they'll ship it free, including the 61 lb stone!

No. Calipers are not as accurate as micrometers because they spring
more and provide less "feel". Their value is wide range and quick
operation for ballpark measurements to within a coupla thou. That's
often quite sufficient, certainly in roughing or intermediate
operations.

I've found the Asian import micrometers to be very good value, and
some come with a reference with which to check them. Don't know about
sources in the UK but an Asian 3-4" mike can be bought here for about
$35. I have some good mikes (Etalon, Starrett, Fowler) and some Asian
imports. They agree to well under .001", usually to better than half
a thou. The better mikes do have better ratchets or friction
thimbles. I prefer a friction thimble, not found on cheap mikes.

If you want to measure accurately to .0002" or better, go name brand
from Switzerland, Japan or USA in that order. If .001" is close
enough, about any mike with carbide faces will suffice.

That was about the accuracy I was guessing. As calipers go, these are
about as stiff as you can get. Probably weigh between 1 and 2 lbs. But
obviously there's no mechanism to ensure a constant closing force, like
a micrometer thimble.

I figure that as I'm not reboring an engine (I just want to know how
severe the wear on each part is, so I can decide what to replace), a
measurement to a couple of thousandths is good enough for me. After all,
the smallest oversize piston I've seen is +0.010". But if anyone thinks
this is a bad plan, do let me know.

I've never seen a micrometer without a thimble. My 0.0001" Tesa
micrometer (Swiss I think) has a friction thimble and my cheap Draper
metric micrometer still has a ratchet thimble.

What do people think of micrometers with interchangeable anvils, to give
several measurement ranges? So for example you have four anvils which
fit a 4" micrometer frame to give measuring ranges of 0" to 1", 1" to
2", 2" to 3" and 3" to 4"? My gut reaction is that they won't be as
consistent as a single range micrometer, but for brand new ones,
Mitutoyo's website claims this isn't the case.

If calipers are good to a couple of thousandths, my inclination is to
use them for the moment.

I'd avoid them unless you really need them. Full disclosure -- Mitutoyo was
my client for many years, and I wrote all of their articles during those
years and some of their instructional materials. They're quite honest about
what they claim. Just be aware that things like interchangeable-anvil
micrometers sacrifice some assurance for the sake of convenience. They can
be as accurate as any mike; you just have to be a bit more careful.

They *should* come with standards for zeroing the micrometer
after changing the anvils -- as many standards as you have anvils.
These should be enough to assure accuracy at the zero point at least,
and you can use gauge blocks to check for errors in mid spindle travel,
but this should not be a problem with new micrometers anyway -- only
after a lifetime of use by you, or someone else.

Indeed -- you need to develop a feel for the sliding force on
the caliper jaws once closed. (And, of course, the jaws are not carbide
faced, so they will wear more rapidly than most micrometer jaws.)

Probably adequate -- especially as you are closing the calipers
to a given setting (using the adjustment screw with the extra traveling
head clamped down) and then using it as a reference for the dial gauge.
If you had a set of gauge blocks, you could close it to a light sliding
fit on those (to free yourself from the possible errors in the vernier
and in reading it) and hold it in a vise to eliminate the problems from
hand heat causing expansion of the beam.

My first one from about 1960 (long gone, and I don't remember
the brand) had only a small diameter spinner, not a ratchet or friction
thimble, and I developed a feel for letting my fingers slip on the
knurled portion of the man thimble.

I've got others with various designs, including a 0-6" Brown &
Sharpe set which still have no torque limiting features, along with
others with very good friction thimbles.

Remember that you have to zero them against a standard each time
you change the anvils -- or for that matter if you pick it up a week
later and want to measure -- the room temperature might have changed
between the zeroing and the time of the intended measurement.

My main consideration of the problems with the multi-range
micrometers is that you have the large C frame so there is more metal to
expand as your hand warms it, thus more error from the thermal
expansion.

If you can prevent this (including hand warming of the standards
you use when you zero the micrometers) you should be OK. Often a bench
clamp stand for the micrometer is indicated.

And potentially good for better than that as you will be using
them in zeroing the bore gauge. If you use gauge blocks to set the
calipers, you will be better than the caliper.

But then, if you have a stack of gauge blocks, and two longer
blocks at the ends, and you have a setting reference which won't need
the calipers at all. There are devices designed for just this purpose
-- a rectangular tube to hold the blocks, a pair of long blocks for the
ends, and provisions for clamping it down firmly.

The reason is thermal expansion of the caliper. If you're pushing for the
best accuracy, and particularly if the room is cold, holding it in your hand
for a few minutes can make a difference. If you warm a three-inch-long piece
of steel by 30 deg. F, from your 40 deg. shop to 70 deg., it will expand by
almost 0.001". If the piece you're measuring is the same temperature as the
caliper, and if they're both steel, the actual temperature matters little,
but a difference in their temperatures, if it's more than a few degrees, can
result in inaccurate measurement.

But it's time to back up. I suppose you realize that a slide caliper is not
a high-accuracy gage. It's usually used for moderate-accuracy work. You were
asking what accuracy you can achieve with it, and the answer is, using a
good caliper, cotton gloves, and calibration with gage blocks, you probably
can achieve +/- 0.001". But not everyone does. Some people have the touch
and some don't.

You'll be able to achieve that more easily with a micrometer. With good
technique and a good mike, you can cut that range in half. You won't need
gloves if you just hang your pinky or two fingers over the bridge part, or
if you work quickly.

That's a good question. If you need a mike, you'll want both. There will be
differences of opinion about this but my vote, if you already have a
caliper, would be for the gage blocks first.

But it depends on whether you're measuring *relative* dimensions or *absolute* dimensions. If it's the former, you don't need a well-calibrated
gage. If it's the latter, you'll never know for sure how accurately your
gages measure unless you check them from time to time against a gage block
(or a stack) that's somewhere in the middle of the gage's range, and another
one near its largest opening. For a 3" - 4" mike, I'd want at least a 3"
block (or a stack to make that dimension) and a 4" block or stack. I'd like
to have a 1/2" block, too, to wring with the 3" block to test the midrange.

If any of this is unclear, ask, and I or someone else will explain. FWIW,
most home-shop work doesn't require accurate measurement of absolute
dimensions. Usually we're trying to make two things fit together, and what
you need to know for that is their *relative* dimensions. If that's the
case, forget the gage blocks for now.

Thanks. That's the figure I wanted. These are good calipers and I think
my touch is fair, but I'm not being careful about temperature and the
scale on the calipers cannot be moved to calibrate them. Sounds like my
guess of +/- 0.002" is probably fair.

What's a stack?

In this case, I think it's the absolute dimensions. The engine's piston
wobbles noticeably in the bore. It's an aluminium piston in a cast iron
bore, so I suspect there could be much more wear on the piston than the
bore (the bore looks good, without a ridge at the top). So I want to
know how well a brand new piston will fit in the existing bore, before I
buy one.

Multiple gage blocks "wrung" together, which add up to their combined
dimension. If you wring three 1" gage blocks together, they'll equal a
single, 3" block. The wringing technique excludes air between them, and adds
less than 2 millionths of an inch to the stack height.

This only works with good-quality gage blocks. Some of the other setting
tools that have been recommended in this thread can't be stacked up with
that kind of accuracy.

The ideal accuracy of dimensional standards, such as gage blocks, is ten
times or more the accuracy of what you're trying to measure. In other words,
if you're trying to determine the accuracy of your micrometer to +/- 0.0001
in., you ideally should have gage blocks that are accurate to +/- 0.000,01
in. That's a workshop-quality gage block, grade A or a good quality grade B,
which today is called a "grade 3" (or AS-1) in new blocks.

If you're going metric, the picture is a little different.

Having said all that, a cheap workshop-grade gage block will be more
accurate than you need for most shop work. But the troubles with cheap sets
are twofold: they have parallelism problems, as well as looseness in
absolute dimensions; and they often won't wring.

Back to Earth: For what you want, almost anything will do. <g> Any decent
gage standard is a whole lot better than nothing. But you have at least one
mike, and you might want to use it for more demanding work at some time. For
that, get a few gage blocks.

It would really help to have a good engine man chime in here. There are
several around; try a new thread with a title like "Measuring engine bores"
or something like that. You may drag one up.

Many of us can help you with handling gages but engine cylinders have other
issues, like taper and ovality, etc. If it's an old engine, the pistons may
be cylindrical. If it's a newer one, they're probably elliptical ("oval").
And they're tapered along their lengths, too.

In engine work, you have to know not only how to measure, but what it is
you're really measuring. It's not difficult but you do have to know what
you're doing. You want to ask someone who really knows his stuff.

Might do that, but I'm going to try taking the measurements first. I
have the urge to get into the workshop :-).

Interesting. Why are pistons intentionally made elliptical? I've heard
of pistons where the top land is of a smaller diameter (apparently it
reduces wear because the lubrication is poorer at the top), but I've not
heard of an elliptical piston. I'm surprised it doesn't increase the
wear on the sides due to the reduced surface area, and I'd have thought
it could allow the piston to vibrate in an angular sense about the
gudgeon pin. Any more information, Ed?

Because the thicker sections, which are the boss areas for the wrist pins
(piston pins) expand with much more force than the thin sections. So the
pistons have a smaller diameter across the boss area.

Almost all production automobile pistons made today are elliptical. In fact,
they're often elliptical with the major axis in one direction at the top of
the piston, and in the other direction at the bottom. The bottom ellipse is
for better sealing, to meet emission requirements. It has to do with
differential friction and heating between the neutral axis, which is
parallel to the crankshaft, versus the other axis.

When I was at Wasino we had some drawings from Ford that actually had three
different ellipses along their length, from top to bottom, and they had to
blend into each other.

If you don't get an expert to chime in here, I'll see what I can dig up for
you. There is one guy who stops in here from time to time who is an engineer
for one of the world's top piston manufacturers; you won't get any better
info than that from him.

If you're eager to search on it yourself, try both "elliptical piston" and
"oval piston." They're often, incorrectly, called "oval pistons" in the
trade.